Enabling files_texteditor breaks favicon selection

[mmccarn]

Steps to reproduce

• Install owncloud 9.0.0.19
• enable the example theme in config/config.php
  'theme' => 'example',
• observe that apple-touch-icon-precomposed is served from the theme folder:

<link rel="apple-touch-icon-precomposed" href="/themes/example/core/img/favicon-touch.png">

• enable files_texteditor

Expected result

• apple-touch-icon-precomposed should still be served from the theme folder

Observed result

• apple-touch-icon-precomposed is now served from /core/img

<link rel="apple-touch-icon-precomposed" href="/core/img/favicon-touch.png">

Notes:

• The same issue applies to <link rel="shortcut icon" href="/themes/example/core/img/favicon.ico">, but that is already misbehaving due to a separate issue favicon.ico from custom theme is not applied #23758
• Disabling files_texteditor did not fix the favicon issue, even after restarting apache and running sudo -u www-data php occ maintenance:repair. I had to remove the entire files_external folder from /apps/ before the theme icon selection started working again.
• Other theme graphics were not disturbed